1. Field of the Invention
The present invention is directed to a method and apparatus for improving the accuracy of a conventional tape servo track seek algorithm, of the type used to position a write/read head relative to a magnetic tape on which data and servo tracks are present in a high density format.
2. Description of the Prior Art
It is well known that imperfections in the rotating parts of a tape cartridge, such as the capstan wheel and the rotor of the motor, cause characteristic frequencies which can be observed in the tape transversal movements. At a given tape speed and a given length of tape wound on the cartridge hubs, each rotating part has its own characteristic frequency. This causes both longitudinal and transversal vibrations of the tape. These dynamic transversal tape movements have heretofore been characterized in the frequency domain with the aid of spectrum analyzers or by taking the Fast Fourier Transform of time-domain waveforms captured on digital storage oscilloscopes. It is also well known that certain abrupt transversal displacements of the tapes position in the tape guides can take place. This last type of tape displacement is not directly caused by or related to the characteristic frequencies of the rotating parts. This is an essential point of observation for the invention. However, it is not unreasonable to believe that cartridge vibration or external forces (e.g., gravity forces on the heavy tape hubs or external mechanical noise) can trigger these abrupt displacements. The abrupt changes can take place simply because there is room for them to occur. Due to mechanical tolerances of tape guides and tape widths, the tape can not be kept in an absolute position with respect to one side of the tape guide. The worst-case "gap" between the widest guide and the narrowest tape for the quarter-inch cartridges in use today is 46 .mu.m. If an abrupt, change take place randomly, the tape will still be subject to exactly the same vibrations stemming from the rotating parts.
For tapes with data and servo tracks thereon in a high density format, it is critical to be able to accurately position the write/read head relative to a selected track. This is accomplished using a servo head and track seek algorithm, of which many are known. All such algorithms depend for their accuracy on the tape itself being located, and remaining, at an "expected" position in a direction perpendicular (transverse) to the direction of tape travel. The aforementioned transverse tape movements thus degrade, or limit, the accuracy of known track seek algorithms since the servo is normally not locked in during the servo track seek.